Hygroscopic Growth of Aerosols and their Optical Properties

Transcription

1 Hygroscopic Growth of Aerosols and their Optical Properties Cynthia Randles Atmospheric & Oceanic Sciences Princeton University V. Ramaswamy and L. M. Russell

2 ! Introduction Presentation Overview! Aerosol Direct Effect! Hygroscopicity, Deliquescence and Efflorescence! Mie Scattering! Organics, Hygroscopicity, and Scattering: Model Results & Comparisons to Measurements! Summary and Conclusions

3 Direct Radiative Forcing by Aerosols Increased Albedo: Negative (-) Forcing Incident Solar (SW) Radiation Decreased Albedo: Positive (+) Forcing Re-emitted LW Radiation: Positive (+) Forcing Aerosol Layer COOLING Scatter Solar Radiation WARMING WARMING Emitted Terrestrial (LW) Radiation Absorb Solar Radiation Absorb Terrestrial Radiation

4 How does water uptake affect scattering? ( Water uptake by aerosols is a key uncertainty factor in the aerosol direct effect (IPCC, 2001) )! Water uptake depends on aerosol composition.! Water increases particle size! Larger particles scatter more light! Less light reaches the surface Less energy - + Dry Crystal Droplet Scattering & Absorption

5 Example: Sea Salt Aerosol! Oceans cover 2/3 of the Earth s surface, so production of sea salt is nearly global.! Sea salt accounts for 30 to 75% of all natural aerosols (Blanchard and Woodcock, 1980; Winter and Chýlek, 1997)! Sea salt is the primary contributor to the global-mean clearsky radiation balance over oceans (Haywood et. al., 1999)! NaCl is the principal constituent of dry sea salt aerosol (O Dowd and Smith, 1998; Winter and Chýlek, 1997)

6 Deliquescence and Efflorescence Amount of Liquid Water As RH over a wet particle decreases, crystallization does not occur at At the At deliquescence very low relative humidities, humidity, or DRH, atmospheric the particle aerosols the DRH, but at a much lower efflorescence RH. spontaneously absorbs water, producing a saturated aqueous solution. containing inorganic salts are solid.

7 Hygroscopic Growth of Particles Hygroscopic Growth Factor (HGF): Wet Particle Diameter of Wet Particle HGF at given RH Diameter of Dry Particle at dry RH (40%) RH = D p D p ( RH ) ( RH = 40%) Increasing RH Dry Particle

8 Hygroscopicity and Scattering: Organic-Electrolyte Mixtures Cl - Cl - Cl - Na + Na + Cl - COOHC 3 H 6 COOH COOHC 3 H 8 COOH Na + Cl - COOHC 3 H 8 COOH Na + Na + Na + Cl - COOHC 3 H 8 COOH Electrolyte- Droplet at Organic Increased RH Mixture Scattering & Absorption!Organics alter the scattering and absorption of light by aerosols.! Up to! 50% Organics of total alter fine (Penner the aerosol hygroscopicity mass is composed et. al., 1998). of aerosols. of organics. (Middlebrook (Saxena et. al., et al., 1995). 1998) Cl

9 Mie Scattering! Scattering & absorption of light by homogeneous sphere.!refractive Index (m = n r + ik i )! Extinction, Scattering, & Absorption Cross-Sections (C ext, C scat, C abs ): Amount of energy removed from the beam of incident radiation by the particle. Analogous to geometrical area of the particle. C = C + (units are Length 2 ) ext scat abs! Extinction, Scattering, & Absorption Efficiencies (Q ext, Q scat, Q abs ): The extinction cross-section divided by the cross-sectional area of the particle. Q ext = Q scat! Extinction, Scattering & Absorption Coefficients (σ ep, σ sp, σ ap ): The extinction cross section multiplied by a number density N (units Length -3 ). σ ep D = σ sp + σ ap = π p Q Q C abs ext = ( m, C π D D p ext 2 p 4 ) N ( D p ) dd (unitless) p (units Length -1 )

10 Larger Particles Scatter More Light Q ext $ = 0.55 "m C ext (cm -2 ) Real Refractive Index = Imaginary Refractive Index = 1 x Dp Cext = Qext π Real Refractive Index = = Imaginary Refractive Index = 1 = x 1 10 x D p p ("m) D p ("m) Extinction Particles with cross-section diameters section between increases about For non-absorbing particles, C Q as ext ext #C size 0.1 and scat increases. #Q scat. 1 "m. scatter light most efficiently. scatter light C. A. most Randlesefficiently.

11 Sensitivity to Organic Fraction 100% NaCl 90% NaCl, 10% Glutaric Acid 70% NaCl, 30% Glutaric Acid 50% NaCl, 50% Glutaric Acid 100% Glutaric Acid Note: % is % mass dry particle Clean Marine Increased Organic Mass Polluted Marine (Middlebrook et. al., 1998)

12 Simple Sea-Salt Salt -Organic Mixture Na + Cl - NaCl! Aerosol particles in the atmosphere are often complex mixtures of inorganic and organic compounds (Middlebrook et. al., 1998).!NaCl is the major constituent of sea salt (Ming and Russell, 2001). Glutaric Acid (C 5 H 8 O 4 ) H 2 O Water!On average, less than 10% of fine aerosol organic carbon has been attributed to specific compounds (Rogge, 1993).!Di-acids, such as succinic and glutaric acid, have been found in the atmosphere and in marine aerosols (Necessary et. al., 2001; Kawamura and Gagosian, 1990).

13 Methodology: Models & Assumptions Chemical Composition Cl - Na + Na + COOHC 3 H 8 COOH Refractive Index (Erlick et. al., 2000) Organic-Electrolyte Model (Ming & Russell, 2002) HGF Wet Size Distribution Composite Refractive Index Mie Scattering Model (Bohren & Huffman,1985) Dry Size Distribution -3 )102 dn/dlndp (cm Na + Assumed for each composition Dp ("m) (Shettle & Fenn, 1975) Refractive Index H 2 O (Erlick et. al., 2000) Scattering Coefficient (σ scat )

14 HGF: NaCl & Glutaric Acid HGF RH = D (RH) p D p(rh = 40%) (1) (2) (3) (4) Dry Mass Composition (1) 100% NaCl (2) 90% NaCl, 10% Glutaric Acid (3) 70% NaCl, 30% Glutaric Acid (4) 50% NaCl, 50% Glutaric Acid (5) 25% NaCl, 75% Glutaric Acid (6) 100% Glutaric Acid HGF RH (6) (5) Above At Below Mixing 85% the NaCl RH, DRH 10%, and of NaCl, 30% glutaric the and presence organicsalt of decreases glutaric mixture the organic acid begins the acid 50% reduce to deliquescence decreases take growth up water, the by hygroscopic while relative 4%, the 12%, humidity pure growth and salt 20%, (DRH) factor does respectively. (HGF). to not. 68%.

15 Optical Properties Na + Cl - NaCl Glutaric Acid (C 5 H 8 O 4 ) H 2 O Water! Refractive Indices! NaCl: i(1 %10-7 )! Organics: i(6 %10-3 )! H 2 O: i(1.96 %10-9 ) (Erlick et. al., 2001)! Sodium Chloride and Water are nonabsorbing.! Organics are assumed to be mildly absorbing.! For all calculations $ = 0.55 "m (corresponds to peak of solar radiation).

16 Scattering Varies with Composition σ sp (km -1 x 10-1 ) (Randles et. al., 2004) $ = 0.55 "m 70 D (1) 75 2 p σ sp = π 4 RH Q scat (2) 80 (3) ( m, D (4) 85 p (5) 90 ) N ( D (6) ) dd p 95 p Dry Mass Composition (1) 100% NaCl (2) 90% NaCl, 10% model WSOC (3) 70% NaCl, 30% model WSOC (4) 50% NaCl, 50% model WSOC (5) 25% NaCl, 75% model WSOC (6) 100% model WSOC At 85% Above RH, the 10%, DRH 30%, of and 50% pure model NaCl, WSOC (glutaric organics acid growth reduce with the assumed amount refractive of indices) cause scattering. 8%, 22%, and 37% reduction in scattering Below coefficient. the DRH A non-absorbing of NaCl, aerosol causes 7%, 15%, organics and 21% decrease increase for these mixtures. scattering

17 Modeled f(rh) and Observations f(rh) f ( RH ) = b scat 75 (Randles et. al., 2004) bscat ( RH ) ( RH = 40%) (4) (3) (2) (1) (6) (5) RH Dry Mass Composition (1) 100% NaCl (2) 90% NaCl, 10% model WSOC (3) 70% NaCl, 30% model WSOC (4) 50% NaCl, 50% model WSOC (5) 25% NaCl, 75% model WSOC (6) 100% model WSOC Polluted (McInnes et. al., 1998) Marine (McInnes et. al., 1998) At 85% RH, 10%, Increased organic 30%, content and is consistent 50% model WSOC with more reduce polluted f(rh) by 6%, case. 19%, and 32%, respectively.

18 Combined Surface-Aerosol Reflectance (Chýlek & Coakley, 1974) 2 o Aerosol Layer Surface Albedo a (Ignoring other atmospheric constituents) R 1 ωo ω β o = = a R as (1 a) 2a 1 ωo (1 a) > ωoβ 2a 1 ω (1 a) < ω β 2a o 2 2 a = surface albedo or planetary reflectance; fraction of incoming solar radiation reflected to space R as = total reflectance of the aerosol-surface system & o = σ scat / σ ext = single scattering albedo. & o ' = fraction scattered into backward hemisphere! Change in reflectance at top of atmosphere due to presence of aerosol.! For global average conditions (i.e. integrated over all "), if )R = 0; referred to as global average critical ratio.! Equivalent to a > R as or )R > 0; aerosol layer decreases total system reflectance of system and causes heating.! Equivalent to a < R as or )R < 0; aerosol layer increases total system reflectance of system and causes cooling.

19 Cooling Decreased by Organics (6) WARMING Dry Mass Composition (6) 100% model WSOC (4) 50% NaCl, 50% model WSOC (2) 90% NaCl, 10% model WSOC (1) 100% NaCl (1 ω)/ωβ (2) COOLING Farmland and OceansUrban Areas Deserts Clouds Snow and Ice (Randles et. al., 2004) Global Mean Critical Ratio 0.5 )R = Surface Albedo (a) RH = 85% (1) 0.8 (4) At 85% RH and λ = 550 The nm, presence the addition of glutaric of just 10% acid model results WSOC in a decrease can reduce in cooling clear-sky both radiative because cooling of (a) size by 3 changes orders of magnitude. and (b) the However, addition of for absorbing a non absorbing material aerosol, (i.e. refractive 10% and index 50% model changes) WSOC reduce cooling by 3% and 25%.

20 Summary! Organics alter hygroscopic growth factor (HGF):! Prior to deliquescence of pure salt, slightly enhances growth.! After deliquescence of pure salt, suppress growth.! Scattering coefficients affected in similar manner as HGF since dependent on particle size.

21 Summary! Observations of f(rh)! Observations of f(rh) for polluted air lower than for cleaner marine air.! Modeled f(rh) lowers with increasing organic content as growth is suppressed at higher RH.! Increasing organic carbon content may contribute to lowering of f(rh) in more polluted conditions.! Increased organic content causes less cooling because:! Organics suppress the size of the aerosol, allowing less of the aerosol size distribution to shift into the optically active range.! A mildly absorbing organic will have reduced scattering, reducing the amount of cooling.

22 Conclusions!It is important to know the following properties of organic aerosol in order to determine their impact on climate:!mixing state (internal or external)!effects on hygroscopic growth!refractive Indices!Since WSOC can become associated with both sulfates and sea salt (both of which experience deliquescent behavior), it is likely that the global radiative cooling associated with organic compounds is overestimated because of inadequate accounting of their hygroscopic growth September 20, and 2004 absorption effects.

23 Acknowledgements!The Department of Energy s Global Change Education Program (GCEP).!Jeff Gaffney, Milton Constantine, Mary Kinney, and Pat Shoulders of GCEP for the last 6 years of a great program!!princeton advisors L.M. Russell and V. Ramaswamy!GCEP advisor S.E. Schwartz